U.S. patent number 10,254,016 [Application Number 15/116,976] was granted by the patent office on 2019-04-09 for refrigeration cycle apparatus and method for controlling refrigeration cycle apparatus.
This patent grant is currently assigned to AGC INC., Mitsubishi Electric Corporation. The grantee listed for this patent is AGC Inc., Mitsubishi Electric Corporation. Invention is credited to Yusuke Arii, Takashi Ikeda, Tomotaka Ishikawa, Hiroshi Sata.
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United States Patent |
10,254,016 |
Sata , et al. |
April 9, 2019 |
Refrigeration cycle apparatus and method for controlling
refrigeration cycle apparatus
Abstract
A refrigeration cycle apparatus includes: a low-stage
refrigeration cycle including a low-stage compressor, a low-stage
condenser, a low-stage pressure reducing device, and a low-stage
evaporator, and circulating low-stage refrigerant; a high-stage
refrigeration cycle including a high-stage compressor, a high-stage
condenser, a high-stage pressure reducing device, and a high-stage
evaporator, and circulating high-stage refrigerant; a cascade
condenser exchanging heat between the low-stage refrigerant in the
low-stage condenser and the high-stage refrigerant in the
high-stage evaporator, and a controller. The low-stage refrigerant
is a refrigerant that undergoes disproportionation. The low-stage
refrigerant is maintained at a pressure lower than a
disproportionation pressure at which the low-stage refrigerant
undergoes disproportionation.
Inventors: |
Sata; Hiroshi (Tokyo,
JP), Ishikawa; Tomotaka (Tokyo, JP), Ikeda;
Takashi (Tokyo, JP), Arii; Yusuke (Tokyo,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Mitsubishi Electric Corporation
AGC Inc. |
Tokyo
Tokyo |
N/A
N/A |
JP
JP |
|
|
Assignee: |
Mitsubishi Electric Corporation
(Tokyo, JP)
AGC INC. (Tokyo, JP)
|
Family
ID: |
54143899 |
Appl.
No.: |
15/116,976 |
Filed: |
March 17, 2014 |
PCT
Filed: |
March 17, 2014 |
PCT No.: |
PCT/JP2014/057031 |
371(c)(1),(2),(4) Date: |
August 05, 2016 |
PCT
Pub. No.: |
WO2015/140873 |
PCT
Pub. Date: |
September 24, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170108247 A1 |
Apr 20, 2017 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25B
49/022 (20130101); F25B 7/00 (20130101); F25B
43/00 (20130101); F25B 49/02 (20130101); F25B
2700/21152 (20130101); F25B 2700/1933 (20130101); F25B
2700/21151 (20130101); F25B 2700/195 (20130101); F25B
2400/16 (20130101) |
Current International
Class: |
F25B
43/00 (20060101); F25B 7/00 (20060101); F25B
49/02 (20060101) |
Field of
Search: |
;62/175 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
103348200 |
|
Oct 2013 |
|
CN |
|
103562338 |
|
Feb 2014 |
|
CN |
|
2679933 |
|
Jan 2014 |
|
EP |
|
2001-091074 |
|
Apr 2001 |
|
JP |
|
2010-196951 |
|
Sep 2010 |
|
JP |
|
2013-083407 |
|
May 2013 |
|
JP |
|
2013-160427 |
|
Aug 2013 |
|
JP |
|
2012/114450 |
|
Aug 2012 |
|
WO |
|
2014/038028 |
|
Mar 2014 |
|
WO |
|
Other References
Accelerated Sealed Tube Test Procedure for Refrigerant R22
Reactions, Purdue University, 1972. cited by examiner .
Office Action dated Feb. 1, 2018 issued in corresponding CN patent
application No. 201480075170.4 (and English translation thereof).
cited by applicant .
Extended EP Search Report dated Oct. 17, 2017 issued in
corresponding EP patent application No. 14886660.1. cited by
applicant .
International Search Report of the International Searching
Authority dated Jun. 10, 2014 for the corresponding international
application No. PCT/JP2014/057031 (and English translation). cited
by applicant .
Office action dated Sep. 25, 2018 issued in corresponding CN patent
application No. 201480075170.4 (and English translation thereof).
cited by applicant.
|
Primary Examiner: Crenshaw; Henry T
Attorney, Agent or Firm: Posz Law Group, PLC
Claims
The invention claimed is:
1. A refrigeration cycle apparatus comprising: a low-stage
refrigeration cycle including a low-stage compressor, a low-stage
condenser, a low-stage pressure reducing device, and a low-stage
evaporator, and circulating low-stage refrigerant; a high-stage
refrigeration cycle including a high-stage compressor, a high-stage
condenser, a high-stage pressure reducing device, and a high-stage
evaporator, and circulating high-stage refrigerant; a cascade
condenser configured to exchange heat between the low-stage
refrigerant in the low-stage condenser and the high-stage
refrigerant in the high-stage evaporator; and a controller, the
low-stage refrigerant being a refrigerant that undergoes
disproportionation, wherein the controller is configured to
maintain the low-stage refrigerant at a pressure lower than a
disproportionation pressure at which the low-stage refrigerant
undergoes disproportionation.
2. The refrigeration cycle apparatus of claim 1, wherein the
controller is configured to change a low-pressure side pressure of
the high-stage refrigeration cycle to maintain the low-stage
refrigerant at a pressure lower than the disproportionation
pressure of the low-stage refrigerant.
3. The refrigeration cycle apparatus of claim 2, wherein the
controller is configured to reduce the low-pressure side pressure
of the high-stage refrigeration cycle when a cooling load on the
low-stage refrigeration cycle increases, and the controller is
configured to increase the low-pressure side pressure of the
high-stage refrigeration cycle when the cooling load on the
low-stage refrigeration cycle decreases.
4. The refrigeration cycle apparatus of claim 2, wherein the
controller is configured to control the high-stage compressor to
change the low-pressure side pressure of the high-stage
refrigeration cycle.
5. The refrigeration cycle apparatus of claim 1, wherein the
controller is configured to operate the high-stage compressor while
the low-stage compressor is not operating, thereby maintaining the
low-stage refrigerant at a pressure lower than the
disproportionation pressure of the low-stage refrigerant.
6. The refrigeration cycle apparatus of claim 1, wherein the
low-stage refrigeration cycle includes a low-stage liquid receiver
provided in a passage communicating between the low-stage condenser
and the low-stage pressure reducing device.
7. The refrigeration cycle apparatus of claim 6, wherein the
low-stage refrigerant in the low-stage liquid receiver is cooled
while the low-stage compressor is not operating.
8. The refrigeration cycle apparatus of claim 6, wherein the
low-stage refrigeration cycle includes a check valve provided in a
passage communicating between the low-stage compressor and the
low-stage condenser, and a valve provided in a passage
communicating between the low-stage liquid receiver and the
low-stage pressure reducing device, and wherein the controller is
configured to, when the high-stage compressor is stopped, maintain
a state of operating of the low-stage compressor while closing the
valve and then stop the low-stage compressor to maintain the
low-stage refrigerant at a pressure lower than the
disproportionation pressure of the low-stage refrigerant.
9. The refrigeration cycle apparatus of claim 1, wherein the
low-stage refrigeration cycle includes a pressure relief
device.
10. The refrigeration cycle apparatus of claim 1, wherein the
controller is configured to stop, when at least one of a pressure
and a temperature of the low-stage refrigerant exceeds a reference
value, the low-stage compressor to maintain the low-stage
refrigerant at a pressure lower than the disproportionation
pressure of the low-stage refrigerant.
11. The refrigeration cycle apparatus of claim 1, wherein the
high-stage refrigerant is a refrigerant that allows operating
efficiency of a refrigeration cycle to be higher than that of the
refrigeration cycle using the low-stage refrigerant.
12. The refrigeration cycle apparatus of claim 1, wherein the
low-stage refrigerant contains HFO-1123 refrigerant.
13. The refrigeration cycle apparatus of claim 12, wherein the
low-stage refrigerant is a refrigerant mixture of HFO-1123
refrigerant and a HFC-based refrigerant.
14. The refrigeration cycle apparatus of claim 13, wherein the
HFC-based refrigerant is HFC-32 refrigerant.
15. The refrigeration cycle apparatus of claim 12, wherein the
low-stage refrigerant is a refrigerant mixture of HFO-1123
refrigerant and HFO-1234yf refrigerant.
16. A refrigeration cycle apparatus comprising: a low-stage
refrigeration cycle including a low-stage compressor, a low-stage
condenser, a low-stage pressure reducing device, and a low-stage
evaporator, and circulating low-stage refrigerant; a high-stage
refrigeration cycle including a high-stage compressor, a high-stage
condenser, a high-stage pressure reducing device, and a high-stage
evaporator, and circulating high-stage refrigerant; a cascade
condenser configured to exchange heat between the low-stage
refrigerant in the low-stage condenser and the high-stage
refrigerant in the high-stage evaporator; and a controller, the
low-stage refrigerant being a refrigerant that undergoes
disproportionation, the low-stage refrigerant being maintained at a
pressure lower than a disproportionation pressure at which the
low-stage refrigerant undergoes disproportionation, wherein the
low-stage refrigeration cycle includes a low-stage high-pressure
side pressure detecting unit configured to detect a high-pressure
side pressure of the low-stage refrigeration cycle, and a low-stage
low-pressure side pressure detecting unit configured to detect a
low-pressure side pressure of the low-stage refrigeration cycle,
and wherein the controller is configured to control the
high-pressure side pressure, detected by the low-stage
high-pressure side pressure detecting unit, to be close to a
geometric mean of the disproportionation pressure of the low-stage
refrigerant and the low-pressure side pressure detected by the
low-stage low-pressure side pressure detecting unit, thereby
maintaining the low-stage refrigerant at a pressure lower than the
disproportionation pressure of the low-stage refrigerant.
17. A refrigeration cycle apparatus comprising: a low-stage
refrigeration cycle including a low-stage compressor, a low-stage
condenser, a low-stage pressure reducing device, and a low-stage
evaporator, and circulating low-stage refrigerant; a high-stage
refrigeration cycle including a high-stage compressor, a high-stage
condenser, a high-stage pressure reducing device, and a high-stage
evaporator, and circulating high-stage refrigerant; a cascade
condenser configured to exchange heat between the low-stage
refrigerant in the low-stage condenser and the high-stage
refrigerant in the high-stage evaporator; and a controller, the
low-stage refrigerant being a refrigerant that undergoes
disproportionation, the low-stage refrigerant being maintained at a
pressure lower than a disproportionation pressure at which the
low-stage refrigerant undergoes disproportionation, wherein the
low-stage refrigeration cycle includes a check valve provided in a
passage communicating between the low-stage compressor and the
low-stage condenser, and a valve provided in a passage
communicating between the low-stage liquid receiver and the
low-stage pressure reducing device, and wherein the controller is
configured to maintain a state of operating of the low-stage
compressor while closing the valve and then stop the low-stage
compressor to cool the low-stage refrigerant between the check
valve and the valve, thereby maintaining the low-stage refrigerant
at a pressure lower than the disproportionation pressure of the
low-stage refrigerant.
18. The refrigeration cycle apparatus of claim 17, wherein the
controller is configured to maintain, when the high-stage
compressor is stopped, the state of operating of the low-stage
compressor while closing the valve and then stop the low-stage
compressor to maintain the low-stage refrigerant at a pressure
lower than the disproportionation pressure of the low-stage
refrigerant.
19. The refrigeration cycle apparatus of claim 8, wherein a total
capacity of components providing communication between the check
valve and the valve is greater than a maximum volume of the
low-stage refrigerant in a liquid state at a pressure lower than
the disproportionation pressure of the low-stage refrigerant.
20. A method for controlling a refrigeration cycle apparatus
including: a low-stage refrigeration cycle including a low-stage
compressor, a low-stage condenser, a low-stage pressure reducing
device, and a low-stage evaporator, and circulating low-stage
refrigerant; a high-stage refrigeration cycle including a
high-stage compressor, a high-stage condenser, a high-stage
pressure reducing device, and a high-stage evaporator, and
circulating high-stage refrigerant; and a cascade condenser
configured to exchange heat between the low-stage refrigerant in
the low-stage condenser and the high-stage refrigerant in the
high-stage evaporator, the low-stage refrigerant being a
refrigerant that undergoes disproportionation, the method
comprising maintaining the low-stage refrigerant at a pressure
lower than a disproportionation pressure at which the low-stage
refrigerant undergoes disproportionation.
Description
CROSS REFERENCE TO RELATED APPLICATION
This application is a U.S. national stage application of
PCT/JP2014/057031 filed on Mar. 17, 2014, the contents of which are
incorporated herein by reference.
TECHNICAL FIELD
The present invention relates to a multi-stage cascade
refrigeration cycle apparatus including multiple refrigeration
cycles and a method for controlling the multi-stage cascade
refrigeration cycle apparatus.
BACKGROUND ART
A related-art refrigeration cycle apparatus includes: a low-stage
refrigeration cycle that includes a low-stage compressor, a
low-stage condenser, a low-stage pressure reducing device, and a
low-stage evaporator, and circulates low-stage refrigerant; a
high-stage refrigeration cycle that includes a high-stage
compressor, a high-stage condenser, a high-stage pressure reducing
device, and a high-stage evaporator, and circulates high-stage
refrigerant, a cascade condenser exchanging heat between the
low-stage refrigerant in the low-stage condenser and the high-stage
refrigerant in the high-stage evaporator, and a controller. Such a
refrigeration cycle apparatus uses CO.sub.2 refrigerant as the
low-stage refrigerant (refer to Patent Literature 1).
CITATION LIST
Patent Literature
Patent Literature 1: Japanese Unexamined Patent Application
Publication No. 2001-91074 (paragraphs [0007] to [0013], FIGS. 1 to
4)
SUMMARY OF INVENTION
Technical Problem
In such a refrigeration cycle apparatus, the low-stage
refrigeration cycle may be controlled at or below a pressure of 7.4
MPa, which is the critical pressure of CO.sub.2 refrigerant. If the
refrigeration cycle apparatus uses, as the low-stage refrigerant,
for example, HFO-1123 refrigerant (1,1,2-trifluoroethylene
refrigerant) that allows its pressure range to be lower than that
in the use of CO.sub.2 refrigerant, the safety of the refrigeration
cycle apparatus can be improved. In addition, the pressure
resistance of each component of the low-stage refrigeration cycle
can be reduced, thus reducing the cost of the refrigeration cycle
apparatus.
When the evaporating temperature is 10 degrees C., the condensing
temperature is 45 degrees C., the degree of supercooling is 0 K,
and the degree of superheat is 0 K, the coefficient of performance
(COP) of the theoretical cycle using CO.sub.2 refrigerant is 5.70.
The COP with HFC (hydrofluorocarbon) -32 refrigerant in this
condition is 6.33. The COP with HFC-410A refrigerant in this
condition is 6.06. When the evaporating temperature is -30 degrees
C., the condensing temperature is 45 degrees C., the degree of
supercooling is 0 K, and the degree of superheat is 0 K, the COP of
the theoretical cycle using CO.sub.2 refrigerant is 1.94. The COP
with HFC-32 refrigerant in this condition is 2.13. The COP with
HFC-410A refrigerant in this condition is 1.99 (cited from "SI
Niyoru Jokyu Reito Juken Tekisuto (Advanced Level Examination
Textbook of Refrigeration in SI units)", Seventh Revised Edition,
Japan Society of Refrigerating and Air Conditioning Engineers). In
other words, the COP of the theoretical cycle using CO.sub.2
refrigerant as the low-stage refrigerant may be lower than that
using a HFC-based refrigerant as the low-stage refrigerant. If the
above-described refrigeration cycle apparatus uses, as the
low-stage refrigerant, for example, HFO-1123 refrigerant that
allows the COP of the theoretical cycle to be substantially equal
to that using, for example, a HFC-based refrigerant, the operating
efficiency of the refrigeration cycle apparatus can be
improved.
Furthermore, if the low-stage refrigerant used is, for example,
HFO-1123 refrigerant that has a global warming potential (GWP)
lower than or substantially equal to that of CO.sub.2 refrigerant,
the effect of the refrigeration cycle apparatus on global warming
can be reduced.
However, HFO-1123 refrigerant is a refrigerant that undergoes
disproportionation, and a technique for operating a refrigeration
cycle apparatus using such a refrigerant as low-stage refrigerant
has not been established. There is little possibility of, for
example, improved safety of a refrigeration cycle apparatus using
such a refrigerant as low-stage refrigerant, reduced cost of the
apparatus, improved operating efficiency of the apparatus, and
reduced effect of the apparatus on global warming.
The present invention has been made in view of the above-described
problems. An embodiment of the present invention aims to establish
a technique for operating a refrigeration cycle apparatus using, as
low-stage refrigerant, a refrigerant that undergoes
disproportionation and provide a refrigeration cycle apparatus with
increased possibility, achieved by the above-described technique,
of improved safety, reduced cost, improved operating efficiency,
reduced effect on global warming and so on. Another embodiment of
the present invention aims to provide a method for controlling such
a refrigeration cycle apparatus.
Solution to Problem
A refrigeration cycle apparatus according to one embodiment of the
present invention includes: a low-stage refrigeration cycle
including a low-stage compressor, a low-stage condenser, a
low-stage pressure reducing device, and a low-stage evaporator, and
circulating low-stage refrigerant; a high-stage refrigeration cycle
including a high-stage compressor, a high-stage condenser, a
high-stage pressure reducing device, and a high-stage evaporator,
and configured to circulate high-stage refrigerant; a cascade
condenser configured to exchange heat between the low-stage
refrigerant in the low-stage condenser and the high-stage
refrigerant in the high-stage evaporator; and a controller, the
low-stage refrigerant being a refrigerant that undergoes
disproportionation, the low-stage refrigerant being maintained at a
pressure lower than a disproportionation pressure at which the
low-stage refrigerant undergoes disproportionation.
Advantageous Effects of Invention
In the refrigeration cycle apparatus according to this embodiment
of the present invention, the low-stage refrigerant is maintained
at a pressure lower than the disproportionation pressure of the
low-stage refrigerant. Although the low-stage refrigerant is a
refrigerant that undergoes disproportionation, the refrigeration
cycle apparatus can be operated as if the low-stage refrigerant
were not a refrigerant that undergoes disproportionation. This
increases the possibility of, for example, improved safety of the
refrigeration cycle apparatus, reduced cost of the apparatus,
improved energy-saving performance of the apparatus, and reduced
effect of the apparatus on global warming.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagram explaining the configuration of a refrigeration
cycle apparatus according to Embodiment 1.
FIG. 2 is a diagram explaining another configuration of the
refrigeration cycle apparatus according to Embodiment 1.
FIG. 3 is a graph explaining the properties of HFO-1123 refrigerant
used as low-stage refrigerant of the refrigeration cycle apparatus
according to Embodiment 1.
FIG. 4 is a table explaining the properties of a refrigerant
mixture of HFO-1123 refrigerant and HFO-1234yf refrigerant used as
the low-stage refrigerant of the refrigeration cycle apparatus
according to Embodiment 1.
FIG. 5 is a diagram explaining the configuration of a refrigeration
cycle apparatus according to Embodiment 2.
FIG. 6 is a diagram explaining the configuration of a refrigeration
cycle apparatus according to Embodiment 3.
DESCRIPTION OF EMBODIMENTS
Refrigeration cycle apparatuses according to the present invention
will be described below with reference to the drawings.
In the following description, for example, configurations and
operations are for illustrative purposes only, and should not be
construed as limiting the refrigeration cycle apparatuses according
to the present invention. In the drawings, the illustration of
detailed structures may be simplified or omitted appropriately.
Furthermore, redundant or similar descriptions may be simplified or
omitted appropriately.
Embodiment 1
A refrigeration cycle apparatus according to Embodiment 1 will now
be described.
<Configuration of Refrigeration Cycle Apparatus>
The configuration of the refrigeration cycle apparatus according to
Embodiment 1 will be described below.
FIGS. 1 and 2 are diagrams explaining the configuration of the
refrigeration cycle apparatus according to Embodiment 1.
As illustrated in FIGS. 1 and 2, a refrigeration cycle apparatus 1
includes a two-stage refrigerant circuit including a low-stage
refrigeration cycle 10 and a high-stage refrigeration cycle 30. The
refrigeration cycle apparatus 1 may include three or more
refrigeration cycles.
The low-stage refrigeration cycle 10 includes a low-stage
compressor 11, a low-stage condenser 12, a low-stage expansion
valve 13 that serves as a low-stage pressure reducing device, and a
low-stage evaporator 14, and circulates low-stage refrigerant. For
example, if the amount of refrigerant necessary for the low-stage
refrigeration cycle 10 significantly fluctuates in response to a
change in operating conditions, a low-stage liquid receiver 15 may
be provided in a pipe providing communication between the low-stage
condenser 12 and the low-stage expansion valve 13, as illustrated
in FIG. 2. The low-stage expansion valve 13 may be any other
pressure reducing device, such as a capillary tube. The low-stage
evaporator 14 is used as a cooling energy source. The low-stage
refrigerant is a refrigerant that undergoes disproportionation,
such as HFO-1123 refrigerant.
The high-stage refrigeration cycle 30 includes a high-stage
compressor 31, a high-stage condenser 32, a high-stage expansion
valve 33 that serves as a high-stage pressure reducing device, and
a high-stage evaporator 34, and circulates high-stage refrigerant.
The high-stage compressor 31 is of a variable capacity type. The
high-stage expansion valve 33 may be any other pressure reducing
device, such as a capillary tube.
The low-stage condenser 12 and the high-stage evaporator 34 are
included in a cascade condenser 40. In the cascade condenser 40,
the low-stage refrigerant in the low-stage condenser 12 exchanges
heat with the high-stage refrigerant in the high-stage evaporator
34.
The high-stage refrigerant is, for example, an HFC-based
refrigerant that has a high GWP. Since the high-stage refrigeration
cycle 30 has a structure less likely to leak the high-stage
refrigerant such that the high-stage evaporator 34 is included in
the cascade condenser 40, the environment is little affected by the
use of such a refrigerant. In addition, HFC-based refrigerants
provide higher COPs than those provided by other refrigerants, and
thus allows improvement of the operating efficiency of the
high-stage refrigeration cycle 30. The high-stage refrigerant may
be any other refrigerant that has a higher GWP than HFC-based
refrigerants. For example, HFO-1234yf refrigerant
(2,3,3,3-tetrafluoropropene refrigerant), a HC-based refrigerant,
CO.sub.2 refrigerant, or water may be used. In other words, the
high-stage refrigerant is a refrigerant that allows the operating
efficiency of a refrigeration cycle to be higher than that of the
refrigeration cycle using the low-stage refrigerant.
If the high-stage refrigerant is a refrigerant having a high
critical point, such as a HFC-based refrigerant, a high-stage
liquid receiver may be provided on a high-pressure side of the
high-stage refrigeration cycle 30 so that an excess of refrigerant
can be processed. If the high-stage refrigerant is a refrigerant
having a low critical point, such as CO.sub.2 refrigerant, a
high-stage accumulator may be provided on a low-pressure side of
the high-stage refrigeration cycle 30 so that an excess of
refrigerant can be processed.
The low-stage refrigeration cycle 10 further includes a low-stage
high-pressure side pressure sensor 21, serving as a low-stage
high-pressure side pressure detecting unit that detects a
high-pressure side pressure in the low-stage refrigeration cycle
10, a low-stage low-pressure side pressure sensor 22, serving as a
low-stage low-pressure side pressure detecting unit that detects a
low-pressure side pressure in the low-stage refrigeration cycle 10,
and a low-stage discharge temperature sensor 23, serving as a
low-stage discharge temperature detecting unit that detects the
temperature of the low-stage refrigerant discharged from the
low-stage compressor 11. The low-stage high-pressure side pressure
sensor 21 is provided in the pipe providing communication between
the low-stage condenser 12 and the low-stage expansion valve 13.
The low-stage low-pressure side pressure sensor 22 is provided in a
pipe providing communication between the low-stage evaporator 14
and the low-stage compressor 11. The low-stage discharge
temperature sensor 23 is provided in a pipe providing communication
between the low-stage compressor 11 and the low-stage condenser 12.
If any of the sensors is not used in an operation which will be
described later, the sensor may be omitted.
The low-stage high-pressure side pressure sensor 21 and the
low-stage low-pressure side pressure sensor 22 may detect the
pressure of the low-stage refrigerant or may detect any other
physical quantity that can be converted into the pressure of the
low-stage refrigerant. In other words, each of the low-stage
high-pressure side pressure detecting unit and the low-stage
low-pressure side pressure detecting unit in the present invention
may be a detecting unit that substantially detects a pressure.
Furthermore, the low-stage discharge temperature sensor 23 may
detect a discharge temperature of the low-stage refrigerant or may
detect any other physical quantity that can be converted into the
discharge temperature of the low-stage refrigerant.
A detection signal of the low-stage high-pressure side pressure
sensor 21, a detection signal of the low-stage low-pressure side
pressure sensor 22, and a detection signal of the low-stage
discharge temperature sensor 23 are input to a controller 50. The
controller 50 controls overall operation of the refrigeration cycle
apparatus 1. The whole or parts of the controller 50 may include a
microcomputer, a microprocessor unit, an updatable component, such
as firmware, or a program module that is executed in response to an
instruction from, for example, a central processing unit (CPU).
<Operation of Refrigeration Cycle Apparatus>
An operation of the refrigeration cycle apparatus according to
Embodiment 1 will now be described.
In the low-stage refrigeration cycle 10, the low-stage refrigerant
is compressed by and discharged from the low-stage compressor 11
and is then cooled by the low-stage condenser 12 in the cascade
condenser 40. After that, the pressure of the low-stage refrigerant
is reduced by the low-stage expansion valve 13. The low-stage
refrigerant, pressure-reduced by the low-stage expansion valve 13,
evaporates in the low-stage evaporator 14 and then returns to the
low-stage compressor 11 through a suction pipe.
In the high-stage refrigeration cycle 30, the high-stage
refrigerant is compressed by and discharged from the high-stage
compressor 31 and then transfers heat and condenses in the
high-stage condenser 32, serving as an air heat exchanger. After
that, the pressure of the high-stage refrigerant is reduced by the
high-stage expansion valve 33. In the high-stage evaporator 34 in
the cascade condenser 40, the high-stage refrigerant,
pressure-reduced by the high-stage expansion valve 33, evaporates
while exchanging heat with the refrigerant in the low-stage
condenser 12. The high-stage refrigerant then returns to the
high-stage compressor 31.
FIG. 3 is a graph explaining the properties of HFO-1123 refrigerant
used as the low-stage refrigerant of the refrigeration cycle
apparatus 1 according to Embodiment 1.
In the case where the low-stage refrigerant is HFO-1123
refrigerant, as illustrated in FIG. 3, high pressures cause the
low-stage refrigerant to undergo disproportionation. A
disproportionation pressure at which the low-stage refrigerant
undergoes disproportionation decreases with increasing temperature.
In other words, if the pressure remains unchanged, the low-stage
refrigerant will undergo disproportionation at high temperatures.
For example, when the temperature is approximately 120 degrees C.,
the low-stage refrigerant undergoes disproportionation at pressures
above 0.7 MPa. When the pressure is 0.7 MPa, the low-stage
refrigerant undergoes disproportionation at temperatures above 120
degrees C. The disproportionation of HFO-1123 refrigerant, serving
as the low-stage refrigerant, is expressed by Reaction Formula (1).
[Chem. 1] CF.sub.2.dbd.CHF.fwdarw.1/2CF.sub.4+3/2C+HF (1)
FIG. 4 is a table explaining the properties of a refrigerant
mixture of HFO-1123 refrigerant and HFO-1234yf refrigerant used as
the low-stage refrigerant of the refrigeration cycle apparatus
according to Embodiment 1.
In the case where the low-stage refrigerant is the refrigerant
mixture of HFO-1123 refrigerant and HFO-1234yf refrigerant, as
illustrated in FIG. 4, the disproportionation pressure can be
increased. Furthermore, a disproportionation temperature at which
disproportionation occurs can also be increased. In other words,
disproportionation can be made less likely to occur than in the
case where the low-stage refrigerant is HFO-1123 refrigerant. As
the molar ratio of HFO-1123 refrigerant to HFO-1234yf refrigerant
decreases, or as the mixture ratio of HFO-1234yf refrigerant to
HFO-1123 refrigerant increases, the disproportionation pressure
rises.
In the case where the low-stage refrigerant is a refrigerant
mixture of HFO-1123 refrigerant and HFC-32 refrigerant, the
disproportionation pressure can be further increased as compared
with that in the case where the low-stage refrigerant is the
refrigerant mixture of HFO-1123 refrigerant and HFO-1234yf
refrigerant. In addition, the disproportionation temperature can
also be further increased.
If the low-stage refrigerant undergoes disproportionation, reaction
products of the disproportionation would accelerate decomposition,
causing an adverse effect on, for example, the operation of the
refrigeration cycle apparatus 1. To reduce a likelihood that the
high-pressure side pressure in the low-stage refrigeration cycle 10
may increase to a value higher than the disproportionation pressure
of the low-stage refrigerant, therefore, the low-stage refrigerant
is preferably the refrigerant mixture of HFO-1123 refrigerant and
HFO-1234yf refrigerant, since the disproportionation pressure of
the refrigerant mixture is higher than that of HFO-1123
refrigerant. The low-stage refrigerant is more preferably the
refrigerant mixture of HFO-1123 refrigerant and HFC-32 refrigerant,
since the disproportionation pressure of this refrigerant mixture
is higher than that of the refrigerant mixture of HFO-1123
refrigerant and HFO-1234yf refrigerant. Assuming that the low-stage
refrigerant is any of these refrigerant mixtures, however, if the
high-pressure side pressure of the low-stage refrigeration cycle 10
rises, disproportionation would occur.
For the above reasons, the high-pressure side pressure of the
low-stage refrigeration cycle 10 in the refrigeration cycle
apparatus 1 is maintained at a lower pressure than the
disproportionation pressure of the low-stage refrigerant.
Examples of implementation will now be described.
All of or some of the examples may be combined.
Example 1
The controller 50 controls an operation state (e.g., a rotation
speed) of the high-stage compressor 31 such that an operating
pressure (low-pressure side pressure) of the high-stage
refrigeration cycle 30 decreases when a cooling load on the
low-stage refrigeration cycle 10 increases, whereas the operating
pressure (low-pressure side pressure) of the high-stage
refrigeration cycle 30 increases when the cooling load on the
low-stage refrigeration cycle 10 decreases. A decrease in operating
pressure (low-pressure side pressure) of the high-stage
refrigeration cycle 30 increases the difference between the
high-pressure side pressure of the low-stage refrigeration cycle 10
and the low-pressure side pressure of the high-stage refrigeration
cycle 30, resulting in a decrease in high-pressure side pressure of
the low-stage refrigeration cycle 10. An increase in operating
pressure (low-pressure side pressure) of the high-stage
refrigeration cycle 30 reduces the difference between the
high-pressure side pressure of the low-stage refrigeration cycle 10
and the low-pressure side pressure of the high-stage refrigeration
cycle 30, resulting in an increase in high-pressure side pressure
of the low-stage refrigeration cycle 10. Controlling the operation
state (e.g., the rotation speed) of the high-stage compressor 31 in
the above-described manner increases or reduces the amount of heat
transferred from the low-stage refrigerant to the high-stage
refrigerant. If the cooling load on the low-stage refrigeration
cycle 10 changes, the high-pressure side pressure of the low-stage
refrigeration cycle 10 can be maintained at a value below the
disproportionation pressure of the low-stage refrigerant.
Example 2
The controller 50 controls the operation state (e.g., the rotation
speed) of the high-stage compressor 31 such that the high-pressure
side pressure detected by the low-stage high-pressure side pressure
sensor 21 is maintained at a value below the disproportionation
pressure of the low-stage refrigerant. Controlling the operation
state (e.g., the rotation speed) of the high-stage compressor 31 in
the above-described manner increases or reduces the amount of heat
transferred from the low-stage refrigerant to the high-stage
refrigerant. If the cooling load on the low-stage refrigeration
cycle 10 changes, the high-pressure side pressure of the low-stage
refrigeration cycle 10 can be maintained at a value below the
disproportionation pressure of the low-stage refrigerant. The
controller 50 may control the operation state (e.g., the rotation
speed) of the high-stage compressor 31 such that the discharge
temperature detected by the low-stage discharge temperature sensor
23 is maintained at a value below the disproportionation
temperature of the low-stage refrigerant.
Example 3
The low-stage refrigeration cycle 10 includes a pressure relief
device that opens when the pressure or temperature of the low-stage
refrigerant increases to a reference value. The pressure relief
device allows the low-stage refrigerant to be maintained at a
pressure below the disproportionation pressure of the low-stage
refrigerant. For example, as illustrated in FIG. 2, the low-stage
liquid receiver 15 is provided with a fusible plug 15a, serving as
a pressure relief device. When the pressure or temperature of the
low-stage refrigerant increases to the reference value, low-melting
part of the fusible plug 15a is molten, thus forming a hole in the
fusible plug 15a. Consequently, the low-stage refrigerant is
maintained at a pressure below the disproportionation pressure of
the low-stage refrigerant. The controller 50 may stop the low-stage
compressor 11 when the high-pressure side pressure detected by the
low-stage high-pressure side pressure sensor 21 increases to a
reference value or when the discharge temperature detected by the
low-stage discharge temperature sensor 23 increases to a reference
value.
Example 4
The controller 50 controls the operation state (e.g., the rotation
speed) of the high-stage compressor 31 such that the high-pressure
side pressure detected by the low-stage high-pressure side pressure
sensor 21 is a geometric mean of the disproportionation pressure of
the low-stage refrigerant and the low-pressure side pressure
detected by the low-stage low-pressure side pressure sensor 22.
Controlling the operation state (e.g., the rotation speed) of the
high-stage compressor 31 in the above-described manner allows the
high-pressure side pressure of the low-stage refrigeration cycle 10
to be an intermediate pressure between the disproportionation
pressure of the low-stage refrigerant and the low-pressure side
pressure of the low-stage refrigeration cycle 10. Consequently, the
high-pressure side pressure of the low-stage refrigeration cycle 10
can be maintained at a value below the disproportionation pressure
of the low-stage refrigerant and an increase in discharge
temperature of the refrigerant discharged from the low-stage
compressor 11 can be suppressed.
In addition, the high-pressure side pressure of the low-stage
refrigeration cycle 10 decreases and the compression ratio of the
high-stage compressor 31 increases, so that the operating
efficiency is improved, thus achieving energy saving in the
refrigeration cycle apparatus 1. In particular, if the high-stage
refrigerant is, for example, a HFC-based refrigerant, energy saving
in the refrigeration cycle apparatus 1 is further improved. For
example, assuming that the outdoor temperature is 32 degrees C. and
the evaporating temperature of the low-stage evaporator 14 is in a
range from -10 degrees C. to -40 degrees C., if the high-stage
refrigerant is HFC-410A refrigerant, the operating efficiency of
the refrigeration cycle apparatus 1 can be substantially
maximized.
<Behavior of Refrigeration Cycle Apparatus>
The behavior of the refrigeration cycle apparatus according to
Embodiment 1 will now be described.
In the refrigeration cycle apparatus 1, the low-stage refrigerant
is maintained at a pressure lower than the disproportionation
pressure of the low-stage refrigerant. Although the low-stage
refrigerant is a refrigerant that undergoes disproportionation,
such as HFO-1123 refrigerant, the refrigeration cycle apparatus 1
can be operated as if the low-stage refrigerant were not a
refrigerant that undergoes disproportionation. This increases the
possibility of, for example, improved safety of the refrigeration
cycle apparatus 1, reduced cost of the refrigeration cycle
apparatus 1, improved energy-saving performance of the
refrigeration cycle apparatus 1, and reduced effect of the
refrigeration cycle apparatus 1 on global warming.
Although, for example, HFO-1123 refrigerant, the refrigerant
mixture of HFO-1123 refrigerant and HFC-32 refrigerant, and the
refrigerant mixture of HFO-1123 refrigerant and HFO-1234yf
refrigerant are refrigerants that undergo disproportionation, these
refrigerants enable the upper limit pressure of the low-stage
refrigeration cycle 10 to be lower than that using CO.sub.2
refrigerant. Consequently, the refrigeration cycle apparatus 1
using such a refrigerant as the low-stage refrigerant can be
operated as if the low-stage refrigerant were not a refrigerant
that undergoes disproportionation. This can improve the safety of
the refrigeration cycle apparatus 1, reduce the pressure resistance
of each component of the low-stage refrigeration cycle 10, and thus
reduce the cost of the refrigeration cycle apparatus 1.
Although, for example, HFO-1123 refrigerant, the refrigerant
mixture of HFO-1123 refrigerant and HFC-32 refrigerant, and the
refrigerant mixture of HFO-1123 refrigerant and HFO-1234yf
refrigerant are refrigerants that undergo disproportionation, these
refrigerants allow the COP of the theoretical cycle to be
substantially equal to that using a HFC-based refrigerant, for
example. Consequently, the refrigeration cycle apparatus 1 using
such a refrigerant as the low-stage refrigerant can be operated as
if the low-stage refrigerant were not a refrigerant that undergoes
disproportionation. This can improve the operating efficiency of
the refrigeration cycle apparatus 1.
Although, for example, HFO-1123 refrigerant, the refrigerant
mixture of HFO-1123 refrigerant and HFC-32 refrigerant, and the
refrigerant mixture of HFO-1123 refrigerant and HFO-1234yf
refrigerant are refrigerants that undergo disproportionation, these
refrigerants have a GWP lower than or substantially equal to that
of CO.sub.2 refrigerant. Consequently, the refrigeration cycle
apparatus 1 using such a refrigerant as the low-stage refrigerant
can be operated as if the low-stage refrigerant were not a
refrigerant that undergoes disproportionation. This can improve the
effect of the refrigeration cycle apparatus 1 on global
warming.
Furthermore, in the case where the low-stage refrigerant is the
refrigerant mixture of HFO-1123 refrigerant and HFC-32 refrigerant
or the refrigerant mixture of HFO-1123 refrigerant and HFO-1234yf
refrigerant, the disproportionation pressure of the low-stage
refrigerant can be made higher than that of HFO-1123 refrigerant
used as the low-stage refrigerant. This increases the reliability
with which the refrigeration cycle apparatus 1 using such a
refrigerant as the low-stage refrigerant is operated as if the
low-stage refrigerant were not a refrigerant that undergoes
disproportionation.
The refrigeration cycle apparatus 1 may be a refrigerating device
or a freezing device, such as a showcase, an industrial
refrigerator-freezer, or a vending machine, required to be free
from chlorofluorocarbons (CFCs) or reduce the amount of CFC
refrigerant used, or achieve energy saving.
Embodiment 2
A refrigeration cycle apparatus according to Embodiment 2 will now
be described.
A description overlapping or similar to that in Embodiment 1 is
simplified or omitted appropriately.
<Configuration of Refrigeration Cycle Apparatus>
The configuration of the refrigeration cycle apparatus according to
Embodiment 2 will be described below.
FIG. 5 is a diagram explaining the configuration of the
refrigeration cycle apparatus according to Embodiment 2.
As illustrated in FIG. 5, the low-stage refrigeration cycle 10
includes the low-stage liquid receiver 15 provided in the pipe
providing communication between the low-stage condenser 12 and the
low-stage expansion valve 13, a check valve 16 provided in the pipe
providing communication between the low-stage compressor 11 and the
low-stage condenser 12, and a solenoid valve 17, serving as a
valve, provided in a pipe providing communication between the
low-stage liquid receiver 15 and the low-stage expansion valve
13.
The high-stage refrigeration cycle 30 includes a cooler 35, serving
as a cooling unit that cools the low-stage refrigerant. The cooler
35 is, for example, a pipe providing communication between the
high-stage expansion valve 33 and the high-stage evaporator 34 in
the high-stage refrigeration cycle 30. For example, the pipe is
disposed so as to extend through the low-stage liquid receiver 15,
thus cooling the low-stage refrigerant in the low-stage liquid
receiver 15.
<Operation of Refrigeration Cycle Apparatus>
An operation of the refrigeration cycle apparatus according to
Embodiment 2 will now be described.
In a normal operation, the controller 50 allows the low-stage
refrigerant cycle 10 to circulate the low-stage refrigerant and
allows the high-stage refrigeration cycle 30 to circulate the
high-stage refrigerant as in Embodiment 1. In some cases, the
low-stage compressor 11 is intermittently operated for temperature
control, for example. When the low-stage compressor 11 is stopped
in such a case, the controller 50 closes the solenoid valve 17 and
continues to operate the low-stage compressor 11 for a
predetermined period of time before the low-stage compressor 11 is
stopped. Such an operation of the controller 50 allows the
low-stage refrigerant in the low-stage refrigeration cycle 10 to be
stored at a high pressure between the check valve 16 and the
solenoid valve 17 in the low-stage refrigeration cycle 10,
particularly in the low-stage liquid receiver 15. The low-stage
compressor 11 is stopped under the above-described conditions.
The controller 50 operates the high-stage compressor 31 while the
low-stage compressor 11 is not operating. Such an operation of the
controller 50 allows the low-stage refrigerant in the low-stage
condenser 12 to be cooled by the high-stage refrigerant in the
high-stage evaporator 34 in the cascade condenser 40. For example,
if the ambient temperature rises, the refrigerant in the low-stage
refrigeration cycle 10 will be maintained at a high density, thus
suppressing an increase in pressure of the low-stage
refrigerant.
In addition, the cooler 35 cools the inside of the low-stage liquid
receiver 15. Since a large amount of low-stage refrigerant is
stored in the low-stage liquid receiver 15, the low-stage
refrigerant is effectively cooled, thus further suppressing an
increase in pressure of the low-stage refrigerant.
<Behavior of Refrigeration Cycle Apparatus>
The behavior of the refrigeration cycle apparatus according to
Embodiment 2 will now be described.
In the refrigeration cycle apparatus 1, when the low-stage
compressor 11 is stopped, the low-stage refrigerant is maintained
at a pressure lower than the disproportionation pressure of the
low-stage refrigerant. Although the low-stage refrigerant is a
refrigerant that undergoes disproportionation, such as HFO-1123
refrigerant, the refrigeration cycle apparatus 1 can be operated as
if the low-stage refrigerant were not a refrigerant that undergoes
disproportionation. This increases the possibility of, for example,
improved safety of the refrigeration cycle apparatus 1, reduced
cost of the refrigeration cycle apparatus 1, improved energy-saving
performance of the refrigeration cycle apparatus 1, and reduced
effect of the refrigeration cycle apparatus 1 on global
warming.
Embodiment 3
A refrigeration cycle apparatus according to Embodiment 3 will now
be described.
A description overlapping or similar to those in Embodiments 1 and
2 is simplified or omitted appropriately.
<Configuration of Refrigeration Cycle Apparatus>
The configuration of the refrigeration cycle apparatus according to
Embodiment 3 will be described below.
FIG. 6 is a diagram explaining the configuration of the
refrigeration cycle apparatus according to Embodiment 3.
As illustrated in FIG. 6, the low-stage refrigeration cycle 10
includes the low-stage liquid receiver 15 provided in the pipe
providing communication between the low-stage condenser 12 and the
low-stage expansion valve 13, the check valve 16 provided in the
pipe providing communication between the low-stage compressor 11
and the low-stage condenser 12, and the solenoid valve 17 provided
in the pipe providing communication between the low-stage liquid
receiver 15 and the low-stage expansion valve 13. The high-stage
refrigeration cycle 30 may include the cooler 35 as in Embodiment 2
or may exclude the cooler 35.
The low-stage liquid receiver 15 has such a capacity that when a
pressure inside the low-stage liquid receiver 15 is lower than the
disproportionation pressure of the low-stage refrigerant, the
entire low-stage refrigerant in a liquid state can be stored
between the check valve 16 and the solenoid valve 17. Specifically,
a maximum volume of the low-stage refrigerant in a liquid state is
obtained based on the total amount of low-stage refrigerant
enclosed in the low-stage refrigeration cycle 10 and the estimated
highest temperature of ambient air. The capacity of the low-stage
liquid receiver 15 is set so that the total capacity of the
components providing communication between the check valve 16 and
the solenoid valve 17 is greater than the maximum volume. The total
capacity of the components providing communication between the
check valve 16 and the solenoid valve 17 is the sum of the capacity
of the low-stage liquid receiver 15 and, for example, the capacity
of the low-stage condenser 12, the capacity of a pipe providing
communication between the check valve 16 and the low-stage
condenser 12, the capacity of a pipe providing communication
between the low-stage condenser 12 and the low-stage liquid
receiver 15, and the capacity of a pipe providing communication
between the low-stage liquid receiver 15 and the solenoid valve
17.
<Operation of Refrigeration Cycle Apparatus>
An operation of the refrigeration cycle apparatus according to
Embodiment 3 will now be described.
For example, when the operation of the high-stage compressor 31 is
stopped due to, for example, a failure of the high-stage compressor
31, the controller 50 closes the solenoid valve 17 and continues to
operate the low-stage compressor 11 for a predetermined period of
time before the low-stage compressor 11 is stopped. Such an
operation of the controller 50 allows the low-stage refrigerant in
the low-stage refrigeration cycle 10 to be stored at a high
pressure between the check valve 16 and the solenoid valve 17 in
the low-stage refrigeration cycle 10, particularly in the low-stage
liquid receiver 15. The low-stage compressor 11 is stopped under
the above-described conditions.
When the operation of the high-stage compressor 31 is stopped, a
heat transfer unit for the low-stage refrigeration cycle 10 is
lost. However, the low-stage refrigerant is stored at a high
pressure between the check valve 16 and the solenoid valve 17 in
the low-stage refrigeration cycle 10, particularly in the low-stage
liquid receiver 15, and is cooled by the ambient air. Thus, the
refrigerant turns into a two-phase gas-liquid state close to a
state of saturated liquid, so that the refrigerant is maintained at
a high density. Consequently, the low-stage refrigerant is
maintained at a low pressure. This eliminates or reduces a
likelihood that the pressure of the low-stage refrigerant may
increase to a value higher than the disproportionation pressure of
the low-stage refrigerant. Additionally, this eliminates or reduces
a likelihood that the pressure of the low-stage refrigerant may
exceed the upper limit pressure, or a design pressure, thus
improving the reliability of the refrigeration cycle apparatus
1.
The low-stage liquid receiver 15 has such a capacity that when a
pressure inside the low-stage liquid receiver 15 is lower than the
disproportionation pressure of the low-stage refrigerant, the
entire low-stage refrigerant in a liquid state can be stored
between the check valve 16 and the solenoid valve 17. This capacity
is determined based on the estimated highest temperature of the
ambient air. If the temperature of the ambient air rises, an
increase in pressure of the low-stage refrigerant caused by an
insufficient total capacity of the components providing
communication between the check valve 16 and the solenoid valve 17
is suppressed. This further eliminates or reduces the likelihood
that the pressure of the low-stage refrigerant may increase to a
value higher than the disproportionation pressure of the low-stage
refrigerant. In addition, the likelihood that the pressure of the
low-stage refrigerant may exceed the upper limit pressure, or the
design pressure is further eliminated or reduced, thus further
increasing the reliability of the refrigeration cycle apparatus
1.
Since the low-stage refrigerant stored between the check valve 16
and the solenoid valve 17 in the low-stage refrigeration cycle 10
is in the two-phase gas-liquid state close to the saturated liquid
state, the pressure of the low-stage refrigerant can be obtained
from the temperature of the low-stage refrigerant. Consequently,
the pressure resistance of part of the low-stage refrigeration
cycle 10 between the check valve 16 and the solenoid valve 17 can
be determined based on a pressure converted from the estimated
highest temperature of the ambient air.
<Behavior of Refrigeration Cycle Apparatus>
The behavior of the refrigeration cycle apparatus according to
Embodiment 3 will now be described.
In the refrigeration cycle apparatus 1, when the high-stage
compressor 31 is stopped, the low-stage refrigerant is maintained
at a pressure lower than the disproportionation pressure of the
low-stage refrigerant. Although the low-stage refrigerant is a
refrigerant that undergoes disproportionation, such as HFO-1123
refrigerant, the refrigeration cycle apparatus 1 can be operated as
if the low-stage refrigerant were not a refrigerant that undergoes
disproportionation. This increases the possibility of, for example,
improved safety of the refrigeration cycle apparatus 1, reduced
cost of the refrigeration cycle apparatus 1, improved energy-saving
performance of the refrigeration cycle apparatus 1, and reduced
effect of the refrigeration cycle apparatus 1 on global
warming.
Although Embodiments 1 to 3 have been described above, the present
invention is not limited to the above description of Embodiments 1
to 3. For example, all or some of Embodiments 1 to 3. Examples 1 to
4, and modifications can be combined.
REFERENCE SIGNS LIST
1: refrigeration cycle apparatus; 10: low-stage refrigeration
cycle; 11: low-stage compressor; 12: low-stage condenser; 13:
low-stage expansion valve; 14: low-stage evaporator; 15: low-stage
liquid receiver; 15a: fusible plug; 16: check valve; 17: solenoid
valve; 21: low-stage high-pressure side pressure sensor; 22:
low-stage low-pressure side pressure sensor; 23: low-stage
discharge temperature sensor; 30: high-stage refrigeration cycle;
31: high-stage compressor; 32: high-stage condenser; 33: high-stage
expansion valve; 34: high-stage evaporator; 35: cooler; 40: cascade
condenser; and 50: controller.
* * * * *